The objective of the proposed work is to study the changes in ultrastructure resulting from (1) restoration of cerebral circulation following brief episodes of ischemia and from (2) focal freezing of the cerebral cortex. Cerebral ischemia is induced by hypovolemic hypotension, and focal freeze lesions are produced by application of dry ice to the exposed dura mater. Electrical activity of the cortex is monitored by means of the electrocardiogram during the period of recovery from the injury and immediately prior to fixation for electron microscopy. Structures which might be involved in the generation of propagation of seizure activity will be analyzed for qualitative and quantitative changes, and these will be correlated with progression of the lesion and with known electrical events. BIBLIOGRAPHIC REFERENCE: Williams, V. 1975. Intercellular relationships in the external glial limiting membrane of the neocortex of the cat and rat. Am. J. Anat., 144: 421-432.